Remote Direct Memory Access (RDMA) enables a host to make a subset of its memory directly available to a remote host. After RDMA connectivity is established between two TCP/IP stacks, either host can write to the memory of the remote host with no involvement from the remote host processors or operating system. RDMA enables efficient communications between the hosts because all the low-level functions are managed by RDMA network interface cards (RNICs) that are connected to each host, rather than by the software stack as is normally done for TCP/IP communications.
RDMA was traditionally confined to high-performance computing (HPC) environments where the cost of maintaining RDMA-capable network fabrics such as InfiniBand was justified given the emphasis of performance over cost. Now that RDMA is available on Ethernet fabrics through standards such as RDMA over Converged Ethernet (RoCE), the cost of adopting RDMA is lower because it can be enabled on the existing Ethernet fabrics that are used for IP network communications. Standard Ethernet management techniques are used to configure the RNIC adapters.
z/OS® Communications Server
provides support for sockets over RDMA by using SMC-R protocols. VTAM® device drivers use Peripheral
Component Interconnect Express® (PCIe)
operations to manage IBM® 10GbE
RoCE Express features that
are defined to z/OS. 
Up to 16 10GbE RoCE Express PFID
values can be defined to a z/OS TCP/IP
stack.